Apple has acknowledged some of its iPhone Duo handsets have developed problems with their screens, just days after the highly-anticipated smartphone went on sale.

Photos posted on social media show lines that appeared on the screens of the technology giant's first ever foldable device, which has a starting price of £1,999 ($2,668) in the UK.

Apple said it was aware of "an issue impacting the inner display on a small number" of the phones and urged affected users to contact the company for a replacement.

The Duo's launch, which attracted crowds at Apple stores around the world last week, is an attempt to reinvigorate the brand's device range and challenge the foldable phones made by rivals.

Some Duo owners also posted photos online showing cracks along the screen's centre crease, while others reported a crunching noise when opening the device.

An iPhone Duo user, Justin Lokos, posted an image of a device with lines across the right side of the display and later uploaded a picture of a new handset alongside the broken one, thanking an Apple store in New York for its help.

The extent of the issue remains unclear. Apple did not say how many devices were affected, while some images showing faulty devices may have been created using AI.

Francisco Jeronimo, technology analyst at market research firm IDC, said that meant some caution was needed before drawing conclusions about the whole production run.

But he added the early reports of issues also showed "that even a company with Apple's engineering resources and supply chain discipline faces a steep learning curve in foldables".